A claw pump includes a pair of rotors which have hook-shaped claws formed thereon and rotate in opposite directions to each other at the same speed in a non-contact manner while maintaining an extremely narrow clearance therebetween inside a housing that forms a pump chamber. The two rotors form a compression pocket, and compressed gas compressed in the compression pocket is discharged through a discharge port. The claw pump continuously performs suction, compression, and exhaust without using a lubricating oil or sealing liquid, thereby producing a vacuum state or pressurized air. As described above, since the lubricating oil or the like is not used, there are advantages that clean gas can be exhausted and discharged, and a higher compression ratio than that of a Roots pump that does not have a compression stroke can be realized.
FIG. 5 illustrates an example of a claw pump according to the related art. In FIG. 5, a claw pump 100 includes a housing 102 that forms a pump chamber therein, and the housing 102 has a cross-sectional shape of two partially overlapping circles. Both end faces of the housing 102 are blocked by side plates (not illustrated), and a suction port 108 is formed in a circumferential wall of the housing 102. Two parallel rotating shafts 110a and 110b are provided inside the housing 102, and rotors 112a and 112b are respectively fixed to the rotating shafts 110a and 110b. The rotors 112a and 112b are provided with hook-shaped claws 114a and 114b which mesh each other in a non-contact manner.
The rotors 112a and 112b rotate in opposite directions to each other (arrow directions), and gas g is suctioned into an inlet pocket P0 that communicates with the suction port 108. Thereafter, two pockets P1 and P2 are formed as the rotors 112a and 112b rotate (see FIG. 5(D)). Furthermore, the two pockets P1 and P2 join and form a compression pocket P (see FIG. 5(F)). In the compression pocket P, immediately after the pockets P1 and P2 join, an initial stage compression space Pe is formed. Thereafter, the initial stage compression space Pe is reduced as the rotors 112a and 112b rotate, such that an end stage compression space Pc is formed. The discharge port 116 is formed in one of the side plates at a position that communicates with the end stage compression space Pc. The gas g is compressed in the compression pocket P and is discharged from the discharge port 116.
In the claw pump, the gas is increased in temperature by compressing the gas, while a higher compression ratio than that of a Roots pump can be realized. The high-temperature gas comes into contact with the surrounding components and increases the temperatures thereof. Therefore, there is concern that contact between the claws of the rotors or contact between the claws and the inner surfaces of the housing may occur due to thermal expansion or deformation and breaking may occur due to insufficient heat resistance. To solve the problems, there is proposed a method of changing the shape of the discharge port or providing a plurality of discharge ports to increase the area of openings, reduce pressure loss, and prevent excessive compression, thereby preventing an increase in temperature. For example, in Patent Literature 1, there is disclosed an example in which discharge ports are formed in both of a pair of side plates that block both end faces of a housing to increase the area of openings.
Otherwise, there has been an attempt to prevent an increase in temperature by reducing a compression ratio through a study of the shape of rotors. For example, in Patent Literature 2, there is disclosed a configuration in which a dent is formed in a face of a convex portion of a female rotor, which faces a claw of a male rotor, and gas in a compression pocket is allowed to escape to the dent when the compression pocket becomes distant from a discharge port, thereby relaxing excessive compression.
In general, a claw pump suctions cooled outside air to obtain a cooling effect. However, in a case where the claw pump is particularly used as a vacuum pump, since the inflow of gas from the suction port is significantly reduced during an operation at a suction pressure of about the ultimate pressure, the cooling effect cannot be obtained. In addition, since the pump chamber is in a vacuum state, a pressure difference from the discharge side occurs, and there is concern that high-temperature gas discharged from the discharge port may flow back to the pump chamber. When the discharge gas that flows back to the pump chamber due to the backflow phenomenon is recompressed while maintaining a high temperature, the temperature thereof is further increased. Accordingly, there may be cases where the temperature of the discharge gas reaches 200° C. to 300° C. As a countermeasure, a method of providing a check valve in the outlet of the discharge port to prevent the backflow of the high-temperature gas is considered.